The present invention relates to a focus adjustment mechanism for a video or image pickup apparatus preferably applicable to a televison camera, a video camera, a digital camera or the like, in which the focus adjustment is performed by changing an optic-axial distance between an imaging element and an image pickup lens.
For example, Japanese Patent No. 7-8021 discloses a conventional focus adjustment mechanism for a video or image pickup apparatus as shown in FIGS. 8 to 10.
An escutcheon 1 constitutes part of a frame of a televison camera. A lens seat 2, mounting a video of image pickup lens (not shown), is provided on a front surface of the escutcheon 1. A focus adjusting ring 3 is rotatably coupled around the outer peripheral portion of the lens seat 2. As shown in FIG. 10, the focus adjusting ring 3 has a cam surface 3a formed on an inner cylindrical surface thereof. The cam surface 3a extends in the circumferential direction while its slope approaches toward the lens seat 2 in the axial direction.
The focus adjusting ring 3 has a pair of holes 3b into which bolts 4a and 4b are inserted. The lens seat 2 has a pair of screw holes 2a opened on a cylindrical portion thereof. When assembled, the focus adjusting ring 3 is overlapped with the lens seat 2 so that the holes 3b of the focus adjusting ring 3 meet the holes 2a of the lens seat 2. Next, the bolts 4a and 4b are inserted through the holes 3b and fixedly engaged with the screw holes 2a. 
After the focus adjusting ring 3 is mounted on the lens seat 2, the focus adjusting ring 3 is rotated relative to the lens seat 2 so that the cum surface 3a of the focus adjusting ring 3 rotates in the circumferential direction while keeping contact with the bolts 4a and 4b securely fixed to the lens seat 2. Thus, the focus adjusting ring 3 advances or retracts in the axial direction with respect to the lens seat 2. In other words, the focus adjusting ring 3 shifts in the protruding direction of the lens seat 2 (i.e., in an optic-axial direction of the image pickup lens).
A bracket 5 is attached to a back surface of the escutcheon 1. The bracket 5 supports a charge coupled device (hereinafter referred to CCD) 6 which is sandwiched between a pressing plate 7 having an aperture opened at the center thereof and another pressing plate 8.
More specifically, the pressing plate 7 is fixed to the pressing plate 8 by means of a bolt 9 so that CCD 6 is exposed from the aperture of the pressing plate 7. Then, the pressing plate 8 is fixed to the bracket 5 by means of a bolt 10. Thus, CCD 6 is securely supported by the bracket 5.
Furthermore, the bracket 5 has a pair of ligulate members 5a and 5b each protruding from an outer cylindrical portion of the bracket 5. A pair of coil springs 11a and 11b contact with the ligulate members 5a and 5b at their axial ends. The coil springs 11a and 11b are pushed at the other axial ends by a chassis 12 constituting part of a main frame. The coil springs 11a and 11b are held in a compressed condition between the chassis 12 and the ligulate members 5a and 5b so that the bracket 5 is resiliently urged toward the escutcheon 1.
Furthermore, the escutcheon 1 has a guide rail (not shown) formed on the back surface thereof. The guide rail extends in the axial direction of the escutcheon 1. The bracket 5 has a protrusion on the outer cylindrical portion. The protrusion of the bracket 5 is shiftable along the guide rail extending in the axial direction of the escutcheon 1. Thus, the bracket 5 shifts with respect to the focus adjusting ring 3 only in the optic-axial direction of the image pickup lens.
According to the above-described conventional television camera, when the focus adjusting ring 3 rotates in one direction, the cam surface 3a moves in the circumferential direction relative to the lens seat 2 while a contact point between the cam surface 3a and each of the bolts 4a and 4b fixed to the lens seat 2 shifts from a deep portion to a shallow portion of the cam surface 3a. Thus, the focus adjusting ring 3 shifts in an axially inward direction relative to the lens seat 2. The bracket 5 departs from the lens seat 2 against the resilient force of the coil springs 11a and 11b. In other words, an optic-axial distance between CCD 6 and the image pickup lens increases.
On the contrary, when the focus adjusting ring 3 rotates in the opposite direction, the cam surface 3a moves in the other circumferential direction relative to the lens seat 2 while the contact point between the cam surface 3a and each of the bolts 4a and 4b fixed to the lens seat 2 shifts from the shallow portion to the deep portion of the cam surface 3a. Thus, the focus adjusting ring 3 shifts in an axially outward direction relative to the lens seat 2. The bracket 5, urged by the coil springs 11a and 11b, approaches toward the lens seat 2. In other words, the optic-axial distance between CCD 6 and the image pickup lens decreases.
In this manner, by rotating the focus adjusting ring 3, the contact point between the cam surface 3a and each of the bolts 4a and 4b can be changed. Thus, the rotational motion of the focus adjusting ring 3 can be converted into a linear or progressive motion of the bracket 5. This makes it possible to vary the optic-axial distance between the image pickup lens and CCD 6 so as to perform the focus adjustment.
However, the above-described conventional focus adjustment mechanism is very complicated in arrangement and its assembling operation is time consuming.